The subject matter disclosed herein generally relates to heat transfer systems and their operation, and more specifically to heat transfer systems utilizing phase change materials.
Phase change materials (PCMs) have been disclosed for use in various applications such as HVAC&R (heating, ventilation, air conditioning and refrigeration) and heat management for electronic components. Phase change materials offer benefits in the area of thermal management due to their high specific heat values compared to non-phase change thermal materials. Many applications of phase change materials have utilized passive heat transfer to and from a phase change material where the phase change material functions as a type of thermal buffer where it either absorbs or releases heat to the surrounding environment depending on the respective temperatures of the PCM and the surrounding environment. Such systems can be effective, but are subject to limitations on the efficiency of heat transfer rates based on effective surface area and thermal transport limitations.
Active heat transfer systems have also been proposed where a heat transfer fluid is pumped or otherwise caused to flow through or past a static PCM. Although such systems can provide heat transfer rate improvements compared to static systems, they often require additional complexity to configure a PCM to maintain a large surface area of PCM in its phase changing state at the interface with the heat transfer fluid. For example, when liquid PCM transitions to solid PCM at the interface with the heat transfer fluid, the solid PCM, having lower thermal conductivity than in liquid state, acts as a thermal barrier between the flowing a heat transfer fluid and the remaining PCM in its liquid state. Additionally, such systems are also subject to efficiency limitations inherent to the heat transfer fluid loop that is used to transfer heat to or away from the phase change material. PCM slurries have been proposed as a way to overcome some of these limitations by allowing the phase change material to be flow as a fluid so that the PCM can be delivered to the location where heat is desired to be delivered or absorbed instead of transferring heat between such a location and a static PCM through the use of a heat transfer fluid. However, PCM slurries have not achieved widespread commercial success due to a number of issues. Typically, PCM slurries contain a non-PCM liquid as a carrier with a PCM material dispersed therein that can transition between a liquid state and a solid state. The PCM material in a PCM slurry is typically contained in microcapsules or is emulsified as micro-droplets/particles in the carrier liquid. The fabrication of PCM-containing microcapsules adds to the complexity and expense of the material, and the microcapsule walls also act as an insulator reducing the thermal conductivity between the fluid and the PCM material. Additionally, materials that undergo any significant volume change during phase change can compromise the structure of the microcapsules after repeated phase change cycles. PCM emulsion slurries can be difficult to maintain as a stable emulsion over repeated phase change cycles, and are limited in the selection of materials that provide effective phase change performance in the desired temperature range and also form a stable emulsion. The requirement for emulsifying agents can also adversely impact both cost and performance of the system. Limitations on the amount of encapsulated or emulsified PCM that can be effectively dispersed in a liquid carrier can also limit the effectiveness of PCM slurries. Additionally, a problem common to many types of PCM materials is that the PCM material often has a limited temperature range over which the phase change occurs, limiting their effectiveness in heat transfer systems that operate over a wide range of temperatures.